1. Field of the Invention
This invention relates to aircraft instrumentation and, more particularly, to devices extending into the air stream for sensing parameters which are used to develop derived air data for instrument readout.
2. Description of the Related Art
It has long been customary to use probes which project from or are mounted to the external surfaces of an aircraft to make various air data measurements which are useful in providing certain instrument displays to a pilot. These probes have been of various types, ranging from the early Pitot tube to multi-aperture, multi-purpose probes mounted at various selected points on the aircraft. One particular probe which is used in certain trans-sonic and supersonic aircraft is disclosed in the De Leo et al U.S. Pat. No. 3,318,146. This is a generally cylindrical probe with a hemispherical forward surface and is designed to be mounted from the side of an aircraft. Other probes of a similar type may have a cylindrical body with a conical or ogival forward surface.
The De Leo et al probe contains a forward facing total pressure (P.sub.T) port, located on the central axis of the probe, and four other ports distributed every 90.degree. around the central axis with the axes of these ports being inclined at substantially 45.degree. from the central axis. A first pair of these ports is located in a central vertical plane, the other pair being located in a central horizontal plane. In this probe, each of the ports is connected to its own separate conduit which leads to a corresponding sensor coupled to a computer in which appropriate scale factors can be supplied for developing desired output information. In straight and level flight, the pressure at the central axis port is total pressure, equal to the normal Pitot or stagnation pressure of the aircraft. The pressure measurement at the central port admittedly varies with aircraft attitude. In this particular instrument, the off axis ports are used to derive measurements of angle of attack (.alpha.) and angle of side slip (.beta.). With the five discrete pressure measurements available from this probe, angle of attack, angle of side slip, total pressure, static pressure and Mach number can be derived.
Mach number is the ratio of the speed of the aircraft to the speed of sound in the fluid medium and is a function, for the De Leo et al probe, of the average pressure of the off-axis ports divided by the total pressure. The relationship between the pressures and Mach number can be expressed as:
For subsonic speeds, EQU P.sub.T /P.sub.S =(1+0.2M.sup.2).sup.3.5 ( 1)
For supersonic speeds, EQU P.sub.T /P.sub.S =(1.2M.sup.2).sup.3.5 [ 6/(7M.sup.2 - 1)].sup.2.5 ( 2)
where M is Mach number, P.sub.T is total pressure and pS is static pressure. P.sub.S can be calculated when P.sub.T and M are known or it can be measured independently by static pressure ports.
While the scale factor and certainty of air flow angle detection of hemisphere probes like that of De Leo et al are superior in performance, the total pressure measurement varies with air flow angle approximately in proportion to a cosine squared function. It is preferable that the total pressure measurement be insensitive to angle of air flow to at least +/-20 degrees.
An alternative probe configuration utilizes the angle insensitive ogival total pressure source in conjunction with angle-of-attack sensing ports which are back on a second conical segment of the cylindrical afterbody. Probes of this type are disclosed in the De Leo et al U.S. Pat. Nos. 4,096,744 and 4,378,696. The characteristic of this differential pressure to angle of attack is much weaker at low angles and becomes non-linear at high angles of attack with a consequent loss of predictability.
The McCormack U.S. Pat. No. 4,718,273 discloses an air data sensor probe for providing measurements of total pressure, static pressure and angle-of-attack for an aircraft in flight. The probe is essentially cylindrical with a hemispherical nose. It has a vertically elongated central opening for measurement of total pressure and a plurality of off-axis openings, manifolded by pairs, to measure angle of attack. Static pressure is measured by a plurality of ports mounted back from the nose, manifolded together to provide the static pressure reading.
According to a paper entitled "Calibration of the Orbiter Air Data System for Space Shuttle Orbital Flight Testing" by Tymms and Hillje which was presented at the 1980 Air Data Systems Conference at the USAF Academy, the Space Shuttles have used a combination probe which provides measurements of total pressure, static pressure and upper and lower .alpha. pressure (for differential .alpha.) from a data probe which is somewhat like those described hereinabove. It has a hemispherical nose containing a central opening for total pressure and a pair of upper and lower ports for .alpha., and a number of ports are located at a station aft on the cylindrical surface supporting the hemispherical nose. However, the total pressure port is sensitive to .alpha. (angle of attack) because it is a small diameter port on a much larger radius surface, thereby sensing the pressure distribution on the surface which varies as the angle of attack changes. Total pressure probes are often made with an opening almost equal to the nearby probe diameter so as to provide an .alpha.-insensitive pressure measurement.
The probes used on the Space Shuttles also have a total temperature sensor mounted thereon. The total temperature and the total pressure openings of the probes used on the Shuttle are entirely independent of, and unrelated to, each other.